The present invention relates generally to a method of synthesising metal alkoxide polymers and relates particularly, though not exclusively, to a method for synthesising hybrid organic/inorganic materials with low optical absorption for optical applications. The invention further relates to the use of these materials for the production of optical waveguides that are used, inter alia, in photonic components for telecommunications networks.
Hybrid organic/inorganic materials, in particular siloxane polymers, are excellent candidates for optical materials, in particular for waveguide applications. These hybrid materials share many of the benefits of polymers including rapid material deposition, low processing temperature and amenability to photolithographic waveguide definition, while the silicate backbone increases the hardness and dilutes the hydrocarbon content. This dilution of the hydrocarbon content is important because overtones from Cxe2x80x94H vibrations cause optical absorption around the 1.3 and 1.55 xcexcm communications bands.
One potential problem with siloxane polymers is Oxe2x80x94H bonds, which also have overtone absorptions around the communications bands and particularly affect the 1.55 xcexcm band. Oxe2x80x94H bonds are a particular problem if the siloxane polymers are produced by the known sol-gel process, and the condensation stage is incomplete. In general, the sol-gel process consists of two stages, namely hydrolysis followed by condensation. Water is used to hydrolyse one or more metal alkoxides to produce M-OH groups that condense to form M-O-M linkages, thereby building up a metal oxide network. For example, the liquid methyl triethoxysilane can according to the sol-gel process be hydrolysed:
CH3Si (OC2H5)3+3H2Oxe2x86x92CH3Si (OH)3+3C2H5OH
And condensed to produce a methyl-substituted silicate:
CH3Si (OH)3xe2x86x92CH3SiO3/2+3/2H2O
The CH3-alkyl substituent is unaffected by the hydrolysis and condensation stages. It will be appreciated that as condensation proceeds, the silicate network becomes increasingly entangled, thereby hindering further condensation reactions, resulting in residual SiOH groups that cause absorption. It is also difficult to completely remove the water from the final product, resulting in additional Oxe2x80x94H absorption. These problems have resulted in the development of siloxane polymers for optical waveguide applications with various methods for minimising the Oxe2x80x94H content. In one example in an aqueous sol-gel system the Oxe2x80x94H content is reduced by incorporating a fluorosilane component and using processing methods that encourage condensation. In another example, a non-aqueous method is used to directly condense silanol and alkoxysilane species and since this method does not involve a hydrolysis stage it is not strictly a sol-gel process.
According to one aspect of the invention there is provided a method of synthesising a metal alkoxide polymer, the method involving the steps of:
acidolysis of a metal alkoxide compound with an acid to produce an intermediate acidolysed solution; and
condensation of the intermediate solution in the presence of a metal alkoxide compound to produce the metal alkoxide polymer.
Generally the metal alkoxide compounds used in the respective acidolysis and condensation steps are different. Alternatively, said metal alkoxides are the same.
Preferably the acidolysis and condensation steps are performed without addition of water. It is to be understood that acid is consumed in the acidolysis reaction of the present invention unlike in the prior art where acid(s) are used to catalyse, and are not consumed in, aqueous hydrolysis reactions.
Preferably the metal alkoxide compounds are organically modified. More preferably at least 25% of the metal alkoxide compounds used in the acidolysis and/or condensation steps are organically modified.
It is to be understood that for the purpose of this specification, an organically modified metal alkoxide compound includes at least one metal to carbon bond that is unaffected during acidolysis and condensation steps.
According to another aspect of the invention there is provided a metal alkoxide polymer being synthesised from acidolysis of a metal alkoxide compound to produce an intermediate acidolysed solution and thereafter condensation of the intermediate acidolysed solution in the presence of another metal alkoxide compound to produce the metal alkoxide polymer.
According to a further aspect of the invention there is provided a metal alkoxide polymer of an optical component, the polymer having a relatively low concentration of hydroxy groups.
Preferably the relatively low concentration of hydroxy groups is less than about 1400 ppm by weight.
Preferably the relatively low concentration of hydroxy groups is reflected in an infra-red (IR) absorption of less than about 140 dB/cm at an MO-H peak near 2760 nm, where M is a metal.
According to yet another aspect of the invention there is provided a method of forming an optical component including a metal alkoxide polymer, said method involving synthesis of the metal alkoxide polymer by acidolysis and condensation of a metal alkoxide compound.
Preferably the acidolysis and condensation step is performed without addition of water.
According to yet a further aspect of the invention there is provided an optical component including a metal alkoxide polymer being synthesised by the acidolysis and condensation of a metal alkoxide compound.
Preferably the optical component is a planar waveguide, optical fibre, integrated device or micro-optic device.
Preferably the metal alkoxide compound(s) have the general formula R1nM(OR)V-n, where: M is a metal of valence V, n is an integer from 0 to (Vxe2x88x921); R is a short chain alkyl group with 1 to 6 carbon atoms; and R1 is an alkyl or aryl group containing from 1 to 20 carbon atoms. The alkyl or aryl group R1 may have substituents including species such as alkenyl, allyl, alkacryloxy, acryloxy, epoxy groups, which can be polymerised either photolytically or thermally to form an organic network, as well as halogen, amino, mercapto, cyano, nitro, amido and hydroxy groups.
If more than one R1 group is present, the R1 groups may or may not be identical. Preferably at least one of the metal alkoxide compounds should have n greater than zero, that is have at least one M-C bond, and said compounds should make up at least 25% of the total number of metal alkoxide species.
Preferably the metal alkoxide compound(s) are alkoxides of silicon, zirconium, titanium, germanium and/or aluminium.
Preferably the acid is an inorganic acid such as boric or phosphoric acid or a carboxylic acid such as formic, acetic or oxalic acid. More preferably the acid is of an element that has a glass forming or glass modifying oxide, and has a pKa greater than about 2.
Preferably the molar ratio of the acid to the metal alkoxide compound in the acidolysis step is from 1:5 to 10:1.
Preferably the acidolysis of the metal alkoxide compound is performed in the presence of a mutual solvent. More preferably the mutual solvent is an alcohol such as methanol.
Preferably the acidolysis and/or condensations steps are each conducted for at least 10 minutes at a temperature of between 0xc2x0 C. and the boiling point of the mutual solvent. More preferably each of said steps is carried out at room temperature for 1 to 24 hours.
Preferably the molar ratio of the metal alkoxide compound in the acidolysis step to the metal alkoxide compound in the condensation step is from 1:10 to 10:1. More preferably said molar ratio is about 1:1.
The acidolysis and condensation steps may be performed repeatedly.
Preferably the metal alkoxide polymer is a resin.